1. Field of the Invention
The present invention relates to photolithographic equipment and, in particular, an exposure apparatus, for use in forming a pattern on a semiconductor substrate. More specifically, the present invention relates to a chromeless photomask and to a method of manufacturing the same.
2. Description of the Related Art
In general, photomasks for use in photolithography are constructed of a transparent mask substrate formed of quartz, for example, and an opaque, metallic light-blocker layer formed of a desired pattern of chromium on the transparent mask substrate. The chromium light-blocker layer shields the exposure light, and causes light passing through the transparent substrate to undergo diffraction and interference, such that a corresponding micro-pattern can be transferred to a photoresist deposited on a semiconductor substrate. Nowadays, though, the density of semiconductor devices is at a very high level as exemplified by 256 or greater MDRAMs. Also, the line widths of the devices are as small as 0.25 μm or less. The photomasks comprising chromium light-blocker layers and that rely on the phenomena of diffraction or interference to transfer a micro-pattern to a semiconductor substrate are limited in their ability to form narrow and accurate micro-patterns. Phase shift masks and chromeless photomasks have been developed as viable alternatives to address these limitations. The phase shift masks or chromeless photomasks use a transparent pattern formed on or in a transparent mask substrate to create a phase difference that produces a photoresist pattern.
A chromeless photomask transmits 100% of light through both a recessed area and a non-recessed area. However, the light passing through the recessed area is out of phase by 180 degrees from the light passing through the non-recessed area. As a result, the light rays passing through the recessed and non-recessed areas produce interference by which a predetermined pattern can be formed on a photoresist deposited on a semiconductor substrate. As long as the photomask pattern is well-designed, a chromeless photomask can be used to create a much finer pattern on a substrate than can be created using a conventional photomask having a chromium light-blocker layer.
An example of a conventional chromeless photomask is shown in FIG. 12. Referring to FIG. 12, a conventional chromeless photomask 1′ includes a core area C including a main pattern 110 of closely spaced areas and a peripheral area P surrounding the core area C as separated a predetermined distance therefrom. The chromeless photomask consists of raised and recessed portions used to create an interference pattern by which a corresponding photoresist pattern can be formed on a semiconductor substrate.
More specifically, the main pattern 110 in the core area C includes a plurality of identical areas which are closely spaced to produce interference when light passes through the core area C, whereby an image corresponding to the main pattern 110 can be effectively transferred to a photoresist. However, neighboring peripheral area P1 surrounding the main pattern 110 may cause problems in the forming of the photoresist pattern. In particular, the outer areas of the main pattern 110 are affected by the neighboring peripheral areas P1 such that an insufficient interference of light occurs at these areas leading, in turn, to a distorted photoresist pattern. In addition, when the photomask 1′ is misaligned along an X-axis, for example, during the exposure process, an undesirable “bar” appears along an area of the photoresist corresponding to one edge of the main pattern 110, and a great amount of pattern distortion occurs along an area corresponding to the opposite edge of the main pattern 110. Moreover, the forming of a photoresist pattern using a conventional chromeless photomask is vulnerable to the problem of lens flare.